30 May 1998 -- Tornado Outbreak in South Dakota

GOES-8 Imager Products:

Severe thunderstorms erupted along a cold front that began to move rapidly eastward across the northern Plains late in the day on 30 May 1998. This convection first produced large hail and tornadoes in South Dakota, including the multiple vortex F4 tornado that killed 6 people and injured over 150 others in Spencer. These storms continued to propagate eastward, developing into a derecho-type of mesoscale convective system that affected parts of the Upper Midwest, the Great Lakes region, and eventually New England.

GOES-8 satellite features: The NOAA GOES-8 visible imagery (above left) covers the time period following the early morning convection over that region, leading up to the initial development of the new convection that eventually spawned the Spencer SD tornado. A curved boundary can be seen migrating southward through the highlighted counties (from 15:45 to about 19:32 UTC); this boundary appears to have been an outflow boundary or gravity wave feature generated by convection over North Dakota early that morning. The western portion of this cloud boundary then appears to surge back to the north from 20:32 to 22:15 UTC, as the cold front begins to propagate eastward toward the Missouri River. Convection develops near the cold front / cloud boundary intersection, and moves toward the affected counties.

Convective initiation often occurs near the periphery of pre-existing wave cloud ("cloud band") features 1. Note the formation of low to middle level wave clouds over north central and northeastern South Dakota (well north of the outlined counties) from about 19:45 to 22:02 UTC; strong convection (producing 1.75 inch diameter hail) develops near the southwestern edge of this wave cloud region at 23:02 UTC. Very subtle wave clouds can also be seen along the extreme western edge of the southward-moving boundary (between 17:02 and 18:15 UTC, just west of the outlined counties).

GOES-8 10.7 micron longwave IR imagery (above right) shows that an enhanced-V / warm wake cloud top signature 2 was first evident just north of Davison county on the 00:55 UTC
image (about 20 minutes prior to the first tornado in Hanson county, and about 50 minutes prior to the Spencer tornado). Another enhanced-V / warm wake signature was apparent on the 23:32 UTC image in northeastern South Dakota, associated with the storm which produced 1.75 inch diameter hail around 23:30 UTC. Note how the coldest cloud tops (orange and red enhancement, -60 to -70 C) over northern portions of Davison and
Hanson counties appear to build back to the southwest after about 01:30 UTC -- this was a result of a cell merger between the original Spencer tornadic supercell and the new convection which was building just to the southwest. Following this cell merger, the enhanced-V / warm wake cloud top signature became larger and more well defined after 02:00 UTC.

GOES-8 Sounder Products:

GOES-8 sounder derived Lifted Index (LI) and Total Precipitable Water (TPW) are shown above. These sounder derived product imagery indicate that the atmosphere over southeastern South Dakota recovered rapidly in the wake of the early morning convection that moved across that region -- the atmosphere remained unstable (LI values of around -8 C, increasing to -12 to -15 C by 23:46 UTC) and had ample moisture (TPW values of 30-40 mm, increasing to 46-48 mm by 23:46 UTC).

Disclaimer: This Web page is intended to present a preliminary quick-look at the meteorological setting of this event, as indicated by GOES satellite and other data; more thorough and detailed analyses of this event will be performed by National Weather Service disaster survey teams and other groups.